Frequency responsive power amplifier



June 15, 1965 w. M MURRAY 3,189,748 FREQUENCY RESPONSIVE POWER AMPLIFIEROriginal Filed April 26, 1960 3 Sheets-Sheet l Fig.3:

I Amplifier Supp/g Voltage Resonant 3'0 Circuit t 30 Curran I x I, to 20 Resonan t Circuit Voltage I William MCMLJF'rag, by )Qw/ 4 m HisAttorney June 15, 1965 w. M MURRAY 3,189,748

FREQUENCY RESPONSIVE POWER AMPLIFIER Original Filed-April 26, 1960 3Sheets-Sheet 2 F/ 'g. 6. Resonant Circuit Amplifier- Supply Volt-ageFm'ng Angle 65 With no Feedback Inventor: Will/am McMuPPay, by )g// d. M

His Attorney- June 15, 1965 w. MCMURRAY 3,189,748

FREQUENCY RESPONSIVE POWER AMPLIFIER Original Filed April 26,

1960 3 Sheets-Sheet 3 L IL F222 za W 12 g6 27 45 as 8/ a2 77 7 84% W x7m 0-4 s/' 1? crvi/ti; 10

Inventor: 2 William McMurmay,

by aha/0% J J His Attorney- United States Patent 3,189,748 FREQUENCYRESPONSIVE POWER AMPLIFIER William McMurray, Bailston Lake, N .Y.,assignor to General Electric Company, a corporation of New York Originalapplication Apr. 26, 1960, Ser. No. 24,743.

Divided and this application May 26, 1%1, Ser.

Claims. (till. 307-41) The present invention relates to a new andimproved power amplifier and to a frequency reference circuit comprisinga part of the power amplifier; and is a division of my copendingapplication Serial No. 24,743, filed April 26, 1960.

More specifically, the invention relates to a power amplifier having anew and improved frequency reference circuit and to the utilization ofsuch circuit in speed control systems, overspeed detectors, and thelike.

Presently available speed control systems for the most part employ somegenerating device such as a tachometer for generating an electricalsignal whose frequency is a function of the speed desired to bemeasured. This signal is then supplied to a frequency discriminatingcircuit that develops an output error signal representative ofdeviations of the frequency of the input signal from a desired value,and th output error signal is then applied through a power amplifier tosome speed controlling device. As a consequence, speed control systemsof this type are relatively complex, and their relative complexitynecessarily involves some sacrifice in their response characteristic.The present invention was evolved to obviate some of the difficultiesinherent in the design of present day speed control systems.

It is, therefore, a primary object of the present invention to provide:a new and improved frequency reference circuit which comprises a partof a power amplifying circuit that has a relatively fast response tochanges in frequency of an input signal supplied to the circuit.

In practicing the invention a power amplifier circuit is provided thatincludes a frequency reference circuit comprised by a resonant circuitwhich includes the primary Winding of a sat-urable transformer and whichis tuned to a desired reference frequency. The power amplifier furtherincludes a silicon controlled rectifier having its control gate elementcoupled to the secondary winding of the saturable transformer. A loaddevice is connected in circuit relationship with the master controlledrectifier, With the circuit thus formed being adapted to be coupledacross a source of alternating current. Connected in circuitrelationship with the master controlled rectifier is a charging devicewhereby the level to which the charging device is charged is determinedby the period of conduction of the master silicon controlled rectifierduring onehalf cycle of the alternating current supply. And a slavecontrolled rectifier is connected in circuit relationship with the loaddevice, and has its gate element opcratively coupled to the chargingdevice whereby the charging device renders the slave controlledrectifier conductive for a time period during the remaining half cycleof the alternating current supply which is dependent upon the charge onthe charging device.

Other objects, features and many of the attendant advantages of thisinvention will be appreciated more readily as the same becomes betterunderstood by reference to the following detailed description, whenconsidered in connection with the accompanying drawings, wherein likeparts in each of the several figures are ident ified by the samereference character, and wherein:

FIGURE 1 is a schematic circuit diagram of a masterslave amplifierconstructed in accordance with the invention, and illustrates a new andimproved frequency 3,189,748 Patented June 15, 1965 reference circuitthat can be employed in driving a master-slave amplifier circuit of thistype;

FIGURE 2 is a schematic circuit diagram of still a different form offrequency reference circuit wherein a portion of the load currentappearing across the output load of an amplifier being controlled by thecircuit is fed back for the purpose of modifying the resonant frequencyof the circuit;

FIGURE 3 is a voltage and current versus time characteristic curve of afrequency reference circuit constructed in accordance with the inventionand illustrates the manner in which the circuit operates to develop acontrol firing pulse for application to a master-slave amplifiercircuit;

FIGURE 4 is a schematic circuit diagram of sitll another form ofmaster-slave amplifier circuit constructed in accordance with theinvention which is controlled by a novel frequency reference circuithaving means incorporated therein for changing the firing angle at whichthe master circuit is fired;

FIGURE 5 is a schematic circuit diagram of another embodiment of amaster-slave power control amplifier controlled by a frequency referencecircuit constructed in accordance with the invention and intended foruse with split inductive loads;

FIGURE 6 is the current-voltage versus time characteristic curve of thecircuit shown in FIGURE 4;

FIGURE 7 is a schematic circuit diagram of an overspeed detector circuitemploying as a I art thereof the novel reference frequency circuit fordeveloping overspeed output signals;

FIGURE 8 is a schematic circuit diagram of an improved form of overspeeddetector circuit similar to that shown in FIGURE 7 which incorporates ameans for maintaining the value of the output signal developed by thecircuit for increasing deviations from a desired reference frequency;

FIGURE 9 is a plot of the amplifier supply voltage versus timecharacteristic for the circuit shown in FIG- URE 1; and

FIGURE 10 is a hysteresis plot of the core of the saturable transformerthat comprises a part of the circuit of FIGURE 1.

FIGURE 1 is the schematic circuit diagram of a speed control circuitemploying as a part thereof a frequency reference circuit comprised by atuned resonant circuit including a capacitor 26 and a linear inductor 27connected in parallel circuit relationship across a source ofalternating current supply 28. The parallel tuned circuit furtherincludes some finite value of resistance indicated by the dotted lineresistor 29 and is connected in series with the primary winding 31 of afirst Isaturable core transformer. The sat-unable core transformer hasits secondary Winding 32 connected through an isolating diode 33 to thecontrol gate element of a master silicon controlled rectifier 11.

The silicon controlled rectifier 11 is connected in a push-pullmaster-slave amplifier circuit which includes a first load device 12connected in series circuit relationship with the master siliconcontrolled rectifier 11 and the primary Winding 13 of a second saturablecore transformer. The series circuit thus comprised is connected acrossa source of alternating current potential 9. A slave silicon controlledrectifier 17 is connected in series circuit relationship with a secondload device 18 across the source 9 in parallel with the first mentionedseries circuit. The gate control element of the slave controlledrectifier 17 is connected to a gate excitation circuit which includesthe secondary winding 14 of the second saturable core transformerconnected in series circuit with a secondary Winding 22 of a voltagestep-down transformer whose primary winding 21 is connected across thesource of alternating =43 current supply 9. The excitation circuitcomprised in part by the two secondary windings l4 and 22 furtherincludes a current limiting resistor 15 and isolating diode in that isconnected to the gate control element of the slave controlled rectifier.I

In operation, the relative phase of the two alternating current supplypotentials and 9 is adjusted to have the relation shown in FIGURE 3 ofthe drawings wherein the dotted curve represents the resonant circuitvoltage 28 and as indicated leads the amplifier supply voltage 9 by 90.it can be appreciated, therefore, that the two alternating currentsupply voltages 9 and 28 ha e the same frequency but are shifted inphase 90 with respect to each other. The resonant circuit is tuned toresonate at the desired operating frequency of the source 23, which, forexample, may be 1660 cycles per second. The current will differ in phasefrom the voltage across this circuit by some phase angle which isdependent upon the actual operating frequency. By appropriate design ofthe first saturable core transformer 31, 32, the current flowing in thereference circuit during each half cycle of the alternating currentsupply voltage 28 will be adequate to drive the core into eitherpositive or negative saturation so that at the end of each half cycle ofcurrent a trigger pulse, indicated at 30, will be produced in thesecondary winding 32. It can be appreciated, therefore, that when thealternating current supply voltage 28 is of precisely the proper phaserelation with respect to the alternating current supply voltage 9, acurrent pulse will be produced at time t which will trigger on themaster silicon controlled rectifier 11 in the push-pull master-slaveamplifier circuit.

The manner in which the master-slave silicon controlled rectifiersfunction as a push-pull power amplifier using the single input controlpulse developed by the frequency reference circuit can best beappreciated in connection with FIGURES 9 and 10 of the drawings. Whenthe polarity of the alternating current supplied to the circuit ofFIGURE 1 is such that the terminal A is positive and the terminal B isnegative, the master controlled rectifier 11 will hold off current flowthrough the load element 12 until such time that a positive controlpulse is applied to the control gate element of the rectifier from thesec-- ondary Winding 32 of the saturable core transformer which firesthe master controlled rectifier. This condition of operation isillustrated schematically by the voltage-time characteristic curveillustrated in FIGURE 9. From an examination of FIGU RE 9, it can beappreciated that from time t to time t the supply potential acrossrectifier 11 will be increasing in a positive direction until at timethe current induced in secondary winding 32 fires the master controlledrectifier ll. Concurrently, it can be appreciated that the supplyvoltage across the slave controlled rectifier 17 is increasing in thenegative direction with respect to its positive electrode, hence, thisrectifier is not conditioned to become conductive. Upon the mastercontrolled rectifier ill. being rendered conductive, load current willbe drawn through the load device 12, and concurrently through theprimary winding 13 of the second saturable transformer. The magnetichysteresis curve of the saturable transformer l3, 14 is illustrated inFIG URE 10 of the drawings, and it is assumed that at time t and t thesaturable transformer is'in its positive saturation condition. Aftertime I, and during the remainder of the period t to t that mastercontrolled rectifier 11 conducts, the current through the primarywinding 13 resets the core of the saturable transformer 13, i4 down theleft side of the curve towards negative saturation. At time 1 thealternating current supply voltage passes through zero so that terminalA thereafter is rendered negative, and conduction through the controlledrectifier 11 will be discontinued. The controlled rectifier 11 thenassumes its blocking condition and resetting of the core of saturabletransformer l3, 14- is arrested at some intermediate point o along thehysteresis curve of the core. Because the polarity of the alternatingcurrent supply voltage reverses at time t the positive electrode of theslave controlled rectifier 17 will be enabled so that this rectifier isin a condition to be rendered conductive. However, the saturabletransformer whose secondary winding 34 is connected to the control gateelement of the slave controlled rectifier 17 will prevent the firing ofthe slave controlled rectifier at this instance due to the fact that itscore has been reset towards negative saturation by the preceding halfcycle of the alternating supply voltage. it is therefore necessary thatthe supply potential applied across the secondary winding 14- firstdrive the core of the saturable transformer back up the right side ofthe hysteresis curve shown in FIGURE 10 into positive saturation beforea positive firing pulse will be applied to the control gate element ofthe slave controlled rectifier 17. From an examination of thevoltage-time characteristic curve of FIGURE 9, it can be appreciatedthat the period of time t to i will be required to again set the core topositive saturation. By a comparison of this time period to the timeperiod t to t it can be appreciated that the time periods are equalbecause the total magnetic flux supplied in resetting the core towardsnegative saturation has to be matched by the total magnetic fluxsupplied in again setting the core to positive saturation before thecore saturates, and supplies a positive polarity gating pulse to thecontrol gate element of the slave controlled rectifier 17. From thepreceding discussion, it can be appreciated that the saturabletransformer l3, 14 in fact constitutes a charging device that is firstcharged to a predetermined level upon the master controlled rectil er llbeing rendered conductive and conducting load current through the loaddevice 12. The charging device then serves to hold off the firing of theslave controlled rectifier 17 during the succeeding half cycle of thealternating current supply potential until such time that the charge ofmagnetic firm on the saturable transformer is offset by an equal valuebut opposite polarity charge applied for an equal period of time, andthereafter, the slave controlled rectifier 1'7 is allowed to conductload current through the load device 18. From an examination of thewaveform shown in FIGURE 9, it can be appreciated that the load currentwill be supplied through the load device 312 for a period of time t; tot during one-half cycle of the alternating current supply potential, andthat during the succeeding opposite polarity half cycle of alternatingcurrent supply potential, load current will be supplied through the loadelement 18 for a period of time 1 to t It can also be appreciated thatthe two periods of time of load current conduction are complementary inthat when summed together they make up a total period of conductionequal to one-half cycle of the operating supply potential. The circuitoperates then in unsymmetrical, pushpull fashion although it is suppliedfrom only a singleended control signal source. Hence, it can beappreciated that the circuit makes available an extremely efficient, lowcost, economical push-pull type amplifier capable of operation' from asingle-ended control source. During operation, the isolating diodes loand 33 serve to augment the control gate element of the controlledrectifiers l1 and l? in .reventing the alternating current supplypotential from prematurely resetting the core of the saturabletransformer 13, 14. Without the isolating diodes 16 and 33 in thecontrol gate element circuit of the controlled rectifiers, leakagecurrent through the gate element may be suflicient to damage thecontrolled rectifiers or to prematurely reset the saturable'transformer13 14. The inclusion of the voltage step-down transformer 21, 22 allowsthe circuit to be operated at lower potential values than wouldotherwise be the case, and hence, allows the circuit to be built fromsmaller and cheaper components.

Referring again to FIGURE 3 of the drawings, as the amplifier supplyvoltage 9 passes through its zero point, conduction through the mastercontrolled rectifier ill will be discontinued and the polarity of thevoltage supplied thereto will be reversed with respect to controlledrectiher 11. Accordingly, upon the current in the frequency 55 referencecircuit passing through its zero in going from the negative half cycleto the positive half cycle, indicated at point 3%, the trigger pulseproduced by the reference circuit will have no effect on the mastercontrolled rectifier 11. Thereafter, the master-slave push-pullamplifier circuit will function in precisely the same manner asdescribed above wherein the saturable transformer 13, 14 will hold offfiring of the slave controlled rectifier 17 for the remainder of theperiod t to While the saturable core transformer is being set towardspositive saturation by the potential being applied from the secondarywinding 22 of the voltage step-down transformer 21, 22. Upon reachingthe point 1 the saturable core transformer 13, 14 reaches positivesaturation indicated at the point (P and a current pulse will beproduced in the gate control circuit which is applied to the gatecontrol element of the slave controlled rectifier 17 causing thisrectifier to be rendered conductive. Conduction through the slavecontrolled rectifier 17 and hence through the load device 18 will thenoccur for the remainder of the negative half cycle of the alternatingsupply potential f indicated to be from t to t The circuit arrangementshown in FIGURE 1 is adapted primarily for use in a speed control systemwherein variations in speed of the element being controlled will appearas a variation in the fre uency of the reference voltage 28. Variationsof this frequency will produce a change in the phase of the current inthe resonant circuit to either advance or retard the phase angle atwhich the master silicon controlled rectifier 11 is fired by the gatingpulse 39 relative to source 9. The resultant change in the phase angleof the firing of the master silicon controlled rectifier 11 results inputting greater or smaller amounts of load current through tie loaddevice 12 to thereby correct for the discrepancy, and to bring thesystem back to its correct speed. Because the frequency referencecircuit is simple in construction, economical to fabricate, and yetentirely reliable in operation, it provides a greatly improved means forsensing changes in frequency which are related to changes in speed in aspeed control system. The reference circuit is ideally adapted for usewith the master-slave unsymmetrical, push-pull amplifier since thelatter is adapted for use with a singleended control system and yetprovides a push-pull power output. Accordingly, the control systemincorporates all of the advantages of both circuits in providing a verysensitive low cost speed control arrangement.

A second form of frequency reference circuit constructed in accordancewith the invention is shown in FIGURE 2. In this arrangement, a seriestuned resonant circuit is formed by a capacitor 26 and a linear inductor27 connected in series circuit relationship across an alternatingcurrent supply voltage source, indicated at 28. The series tuned circuitthus formed is also connected in series circuit relationship with theprimary winding 34 of a first saturable transformer and the twosecondary Winding halves 43 and 44 of an additional saturabletransformer. The secondary winding 35 of the first saturable transformeris connected across a full wave rectifier circuit which is conventionalin construction, and is conprised by two half wave rectifying circuitscoupled together in back-to-back relation across a common load 36. Thehalf wave rectifying circuits are comprised by two silicon controlledrectifiers 37 and 38 with the silicon controlled rectifier 37 beingconnected across one-half 39 of the secondary Winding of an inputtrans-former, and the controlled rectifier 38 being connected across theremaining half 41 of the secondary winding. The primary Winding 42 ofthe input supply transformer has its terminals connected across thesource 9 of alternating current voltage. The source 9 and the referencesignal source 23 have the same operating frequency, but are shifted inphase 90 with respect to each other in the manner illustrated in thecharacteristic curve of FIGURE 3. The circuit is completed by theadditional saturable transformer having split primary and secondarywindings formed by the secondary winding halves 43 and 44 connected inseries circuit relationship with the series tuned circuit 26, 27 and theprimary winding 34 of the first saturable transformer. The split primarywindings 45 and 46 of the additional saturable transformer areinductively coupled to the split secondary windings 43 and 44,respectively, and are connected through a feedback impedance 47 backacross the load device 36 in feedback relationship.

In operation, the circuit arrangement of FIGURE 2 functions in a mannersimilar to that described with relation to FIGURE 1 in that thereference signal supplied from the alternating current source 28 isadjusted so that it leads the supply voltage 9 by and has the samefrequency. The circuit is adjusted so that as the current in the tunedcircuit 2%, 27 including the saturable transformer windings 34, 43, 44passes through zero, the transformer 34, 35 saturates and produces acurrent pulse at 39 which Will provide a positive gating pulse to thecontrolled rectifier 37, for example. In the next succeeding half cycle,the firing pulse 30 will serve to fire the controlled rectifier 38 sothat essentially full wave rectification is accomplished with respect tothe load device 36. The saturable transformer windings 45, 46, which arecoupled back across the load device 36 through the impedance 4'7, serveto feed back a certain portion of the load current into the frequencyreference circuit to thereby vary the tuning of the circuit inaccordance with the load current being drawn. The effect of thisfeedback is to change the frequency at which the circuit resonates. Thecircuit of FIGURE 2 is particularly well adapted for a variable speedcontrol where it is desirable that the circuit operate over a limitedrange of frequencies. Hence, it is extremely well adapted for use with aspeed control system designed to provide for variations of speed withina limited range of speeds. By this arrangement, it is possible toreflect into the frequency of the input signal source 28 a decrease inspeed which will produce an increase in the load current due to theadvance of the phase angle at which the silicon controlled rectifier 37or 38 is fired. In the case where the frequency change is to a highervalue than the mean value to which the circuit is adjusted to operate,then the change will be in a direction to decrease the load currentthrough the load device 36. This increase or decrease in load currentwill be reflected in an increase or decrease in the current fed backthrough the primary windings of the split saturable transformer windings45 and 46, and will result in a change in the reactance that thetransformer reflects into the tuned circuit through the primary windings43, 44. This change in reactance will effectively change the resonantfrequency to which the entire circuit is tuned within the limited rangeof frequencies at which the circuit will operate. By adjustment of theimpedance 47, the amount of current fed back and hence the resonantfrequency of the tuned reference circuit are controlled, therebycontrolling the speed at which the speed control system will operate.

Another form of speed control circuit constructed in accordance with theinvention and which employs negative feedback for speed control purposesis illustrated in FIGURE 4 of the drawings. In the arrangement of FIGURE4, an alternating current source of control signals 28 is coupled acrossa frequency reference circuit comprised by a capacitor 26 and aninductor 27 connected in series circuit relationship with the primarywinding 51 of a saturable core transformer. The secondary winding 52 ofthe saturable core transformer is connected to the control gate elementof a master silicon controlled rectifier 11. The master siliconcontrolled rectifier 11 is connected in series circuit relationship withthe primary winding 53 of a second saturable core transformer and withan inductive load device 54 and current limiting resistor 55,

and this series circuit is connected in parallel circuit relationshipwith a second series circuit formed by a slave controlled rectifier 17,a limiting resistor 56, and an inductive load device 57. The parallelcircuit thus formed has one of its terminals connected to a lowimpedance bias 7 winding 58 that is inductively coupled to the saturabletransformer primary winding 51, and in turn, is connected to one side ofthe alternating current supply 9, and the remaining terminal of theparallel circuit is connected directly to the alternating current source9. The slave controlled rectifier 17 is slaved to the master controlledrectifier 11 through the scondary winding Sil of the second saturabletransformer and an isolating diode 67. To complete the circuit, a pairof commutating diodes 59 and 61 are connected between one end of each ofthe inductive load devices 54, 57, respectively, and the terminal of thealternating current supply source 9 to which the DC. bias winding 58 isconnected.

In operation, the circuit arrangement of FIGURE 4 functions in a mannersimilar to the circuit shown in FIGURE 2 in that the frequency referencecircuit formed by the series tuned capacitor 26 and inductor 27 inseries with the primary winding 51 of the first saturable coretransformer will develop a current pulse in the secondary winding of thesaturable reactor at the end of each half cycle. During the positivehalf cycle of the alternating current supply 9, this current pulseserves to fire the master controlled rectifier 11. The voltage-timecharacteristic curve of the circuit arrangement of FIGURE 4 isillustrated in FIGURE 6 wherein the alternating current supply voltage 9is shown at 62 and the current through the reference circuit isindicated at 63. Initially, where the current passes through a currentzero during the positive half cycle of the alternating current supplyvoltage at point 64-, the master controlled rectifier 11 will berendered conductive. Upon this occasion, load current will be drawnthrough the inductive load device 54 and the primary winding 53 of thesaturable core transformer. This direct current component will passthrough the control winding 58 of the saturable core transformer coupledto the reference circuit so that a small DC. bias is produced on thecore of this transformer. The effect of the DC. bias is to readjust thephase angle at which firing will occur during the following cycle ofoperation. This readjustment will be to a new phase angle position suchas shown at 65, dependent upon the value of the direct currentcomponent, and will be retarded or advanced, depending upon the polarityof the control winding 58. Conduction through the master controlledrectifier 11 will serve to reset the core of the saturable transformer53 towards negative saturation to a point such as shown in FIGURE 10 sothat during the next half cycle of the alternating current supply source9, the core of transformer 53, 50 will have to be set back to positivesaturation prior to firing the slave controlled rectifier 17 in thepreviously described manner. Upon the slave controlled rectifier 17being rendered conductive, load current will be drawn through theinductive load element 57 and limiting resistor 56 and through the DC.bias winding 58. This portion of the load current will serve to bias thecore of the saturable transformer 51 in a direction to advance or retardthe firing angle at which the master controlled rectifier 11 is renderedconductive for succirculate the reactive component of the load currentthrough the load devices 54 and 57, respectively.

Still a different form of speed control circuit constructed inaccordance with the invention is shown in FIG- URE and includes afrequency reference circuit comprised by a series connected capacitor 26and inductor an 27 tuned to series resonance in conjunction with theprimary winding 63 of a first saturaole core transformer. The seriescircuit thus formed is connected across a source of alternating currentreference signals The secondary winding 64 of the saturable coretransformer is, connected to the control gate element of a mastersilicon controlled rectifier 11 which, in turn, is connected in seriescircuit relationship with an inductive load element 54 and limitingresistor 55 across a source of alternating current potential 9. Thejuncture of the inductive load element 54 and controlled rectifier lllare connected through the primary winding of a second saturable coretransformer whose secondary winding 6-5 is connected through anisolating diode 67 to the control gate element of the slave controlledrectifier 17. The slave controlled rectifier i7 is connected in seriescircuit relationship with a current limiting resistor 56 and inductiveload element 57 both of which are paralleled by a commutating diode 61.A

second commutating diode 59 is connected in series with the primarywinding 65 of the second saturable core transformer and a ballastresistor 68 is connected across this circuit. By this arrangement, thereactive component of the load current is recirculated through theinductive load device 54, and is used to reset the saturable transformers5, 66 toward negative saturation in place of the main component of theload current drawn through the controlled rectifier Ill. As aconsequence of this arrangement, the saturable transformer 65, 66 isreset during the period of time prior to conduction of the mastercontrolled rectifier lit, the period from time t to in FIGURE 9. Hence,the period of time from to t during which the saturable transformer 65,66 is set back to positive saturation, which must be equal in durationto the period of resetting action, is in this instance equal in durationto the period of time t to 11. Therefore, the period of time 3 to t inwhich the slave controlled rectifier 17 is conductive is in thisinstance equal in duration to the period of time 2 to t in which themaster controlled rectifier 11 is conductive. Thus, the slaving actionof the slave controlled rectifier 1'7 is symmetrical, instead ofunsymmetrical, with respect to the master controlled rectifier ii. Inother respects, the circuit of FIGURE 5 operates in identical fashion tothe circuit arrangement of FiGURE 4- with the exception that there is nofeedback provided to control the phase angle at which firing of themaster controlled rectifier is accomplished.

FIGURE 7 of the drawings illustrates an overspeed detector which employsthe novel frequency reference circuit as a component part thereof. Thefrequency reference circuit is comprised by a capacitor 26 and linearinductor 27 connected in series circuit relationship with the primarywinding 63 of a saturable core transformer. The series circuit thusformed is tuned to a desired upper limit of the operating frequency, andis connected across a source 28 of input signals having a normaloperating frequency lower than the series resonant frequency of thetuned reference circuit. The secondary winding of the saturabletransformer 64 is connected to the control gate element of a controlledrectifier 71 which is connected in series circuit relationship with aninductive load element 72 and a current limiting resistor '73 across analternating current supply 9 which is in phase with respect to thealternating current supply 28 and has the same operating frequency. Acommutating diode 74 and a ballast resistor '75 are connected inparallel circuit relationship across the load element '72 and thecurrent limiting resistor '73.

The circuit arrangement comprised in the above fashion constitutes anoverspeed detector and functions in the following manner. At the desiredupper limit of the operating frequency, the current and voltage throughthe inresonance frequency reference circuit will be in phase, and sincethe two alternating current sources 9 and 28 have the same frequency andare in phase, the current pulses produced by the saturable transformer63, 64 will occur each time the supply voltage 9 passes through its zerovalue so that the controlled rectifier 71 is not enabled to be renderedconductive. Accordingly, as long as the system with which the overspeeddetector is used maintains its proper speed limit, no corrective actionwill be instituted by the circuit. In the event that the frequency dropsbelow the resonant value, a leading current will be developed in thefrequency reference circuit and the current through the primary winding63 of the saturable transformer 63, 64 will increase through zero inadvance of the positive swings of the supply voltage. Hence, forunder-speed conditions, the polarity of the supply voltage across thesilicon controlled rectifier 71 will be such that the circuit will notbe rendered operative by the positive gating pulse. However, in theevent that the frequency of the source 28 increases above the resonancevalue, indicating an increase in speed of the system with which theoverspeed detector is used above its desired limit value, a laggingcurrent will be produced which will increase through a current zero andthereby produce a triggering pulse subsequent to the supply voltagehaving passed through its zero value and provided a positive enablingpotential across the controlled rectifier 71. On this occasion, thecontrolled rectifier will be rendered conductive, and will supply loadcurrent through a corrective device indicated by the inductive loadelement 72, assumed to be a relay coil or other element used to correctthe overspeed condition.

Because the overspeed detector shown in FIGURE 7 has certain undesirablecharacteristics, namely, that in the event of an overspeed, the maximumvalue of corrective signal occurs at those overspeeds immediatelyadjacent the desired limit speed value, and in the event that theoverspeed condition increases thereafter the corrective value of theload current developed by the circuit drops off in magnitude. In orderto obviate this condition, a second overspeed detector is shown inFIGURE 8 of the drawings which includes a reference frequency circuitformed by a series connected capacitor 26 and inductor 27 and a primarywinding 63 of a saturable core transformer which are series tuned to adesired upper limit of the operating frequency, and are connected acrossthe source of reference signals 28 which is representative of the speedof a device being controlled by the circuit. The saturable transformerhas its secondary winding 64 connected to the gate control element of acontrolled rectifier 71. Controlled rectifier 71, in turn, is connectedin series circuit relationship with an inductive load element 72, suchas the coil of a relay, and limiting resistor 73 across .a source ofalternating current supply 9. Also connected in this series circuit isthe primary winding 78 of a saturable core transformer whose secondarywinding 79 is connected in series circuit with a current limitingresistor 81, a diode 82, and the primary winding 83 of still a thirdsaturable transformer. This third saturable transformer has itssecondary winding 84 connected in reverse relationship through anisolating diode 85 back to the control gate element of the controlledrectifier 71. In this circuit arrangement the alternating supplypotential 9 and reference signal 28 have the same frequency and are inphase so that as long as the frequency is at its preselected limitvalue, which is the resonant frequency of the reference circuit, thecircuit will not be activated. Similarly, if the speed drops below thedesired limiting value, the circuit will not be activated for thepreviously discussed reasons. However, in the event that the speed ofthe device being monitored by the circuit, and hence the frequency ofthe signal 28 increases above the desired limiting value, the currentthrough the reference circuit will lag the potential there-acrosssufiiciently to pass through its current zero at a time when thepotential supplied from the terminals A, B, is positive with respect tothe electrodes of the controlled rectifier 71. Upon this occasion, theensuing gating pulse will render the controlled rectifier conductive forthe remainder of the positive going half cycle of the alternatingcurrent supply potential 9. Conduction through the controlled rectifier7-1 will cause the saturable transformer 78, 79 to be reset down itshysteresis curve to a value such as shown at e in FIGURE 10 of .thedrawings. Thereafter, conduction through the controlled rectifier 71will be terminated as the alternating current voltage from source 9passes through zero value. During the succeeding half cycle of thealternating current potential 9 when the terminal B goes positive, therectifier 82 will be rendered conductive and the core of the secondsaturable transformer '78, 79 will be driven back towards positivesaturation. in doing this, however, it should be noted that voltage willbe held off of the primary winding 83 of the third saturable transformer83, 84 so that the core of the transformer will not be reset towardsnegative saturation, as would normally be the case had controlledrectifier 71 not been fired in the preceding half cycle. Accordingly,during the next succeeding positive half cycle when the terminal A isrendered positive, a positive signal pulse will be applied throughsecondary winding 84 of the third saturable transformer 83, 84 andthrough the diode 85 to the control gate element of the controlledrectifier 71 thereby again turning on this controlled rectifier torepeat the cycle. In this manner all succeeding positive half cycles ofthe alternating current supply potential will result in firing the controlled rectifier 71 and produce successive current flow through theload element 72 thereby providing a steady value corrective signal untilsuch time that the overspeed condition is corrected. Normally, thecircuit will not return to its quiescent condition until the overspeedhas been corrected by appropriate external action, and the circuitmanually reset.

From the foregoing description it can be appreciated that the inventionmakes available a number of new and improved push-pull amplifiers whichemploy silicon controlled rectifiers. These amplifiers are capable ofbeing controlled from a single-ended source of control signals whichincorporate a unique frequency sensitive reference circuit that isespecially adapted for use in speed control systems. Further by simpleadaptation of the basic circuits described, it is possible to vary thereference frequency to which such circuits are sensitive, and to employthe circuits in fabrication of overspeed detectors.

Having described several embodiments of the new and improved push-pullamplifier constructed in accordance with the invention, it is believedobvious that other modifications and variations of the invention arepossible in the light of the above teaching. It is, therefore, to beunderstood that changes may be made in the particular embodiments of theinvention described which are within the full intended scope of theinvention as defined by the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A control circuit including in combination a frequency referencecircuit comprising a resonant circuit tuned to a desired referencefrequency and adapted to be connected to an alternating current source,said resonant circuit including a first saturable transformer havinginductively coupled primary and secondary windings with the primarywinding being connected in series circuit relationship with the resonantcircuit, a master-slave amplifier controlled by said reference circuitand comprising a master silicon controlled rectifier having its controlgate element coupled to the secondary winding of the first sat urabletransformer, a first load device coupled in series circuit relationshipwith the master controlled rectifier with the series circuit thus formedbeing adapted to be connected across a source of alternating current, asecond saturable transformer having inductively coupled primary andsecondary windings with the primary winding being connected in seriescircuit relationship with said load and master controlled rectifier, aslave controlled rectifier and a second load device connected in seriescircuit relationship across the series circuit formed by said mastercontrolled rectifier and said first load device,and a control elementfiring circuit connected in series circuit relationship with thesecondary winding of said second saturable transformer to the controlgate element of the slave controlled rectifier.

2. The combination set forth in claim 1 wherein the resonant circuitcomprises a series tuned circuit and the first saturable transformerincludes an additional winding inductively coupled to the primarywinding thereof and connected in series circuit relationship with theload devices in said master-slave amplifier whereby current through theload devices is fed back to the additional winding to change the averagemagnetic condition of the first saturable transformer to a desired newvalue.

3. A frequency reference circuit comprising a series resonant circuittuned to a desired reference frequency and adapted to be connected to analternating current source and including a saturable transformer havinga primary winding connected in series relationship with the resonantcircuit and having its secondary connected to the input of an amplifier,an additional saturable transformer having its secondary Windingconnected in series circuit relationship with the resonant circuit andthe primary winding of said first saturable transformer and having itsprimary winding connected in parallel circuit relationship With the loadof said amplifier, said amplifier being controlled by said referencecircuit whereby current through the load is fed back to the additionalsaturable transformer to change the resonant frequency of the referencecircuit to a desired new value.

4. A control circuit including in combination a frequency referencecircuit comprising a resonant circuit tuned to the desired referencefrequency and adapted to be connected to a source of alternatingcurrent, said resonant circuit including a first saturable transformerhaving inductively coupled primary and secondary windings with theprimary winding comprising a part of the resonant circuit, a siliconcontrolled rectifier having its gate control element connected to thesecondary winding of said saturable transformer, a load device and acurrent ear/ea limiting resistor connected in series circuitrelationship with said controlled rectifier, the circuit thus formedbeing adapted to be connected across a source of alternating current,and a dioderectifier and ballast device connected in parallel circuitrelationship across said load device.

5. A control circuit including in combination a frequency referencecircuit comprising a resonant circuit tuned to the desired referencefrequency and adapted to be connected to a source of alternatingcurrent, said resonant circuit including a first saturable transformerhaving inductively coupled primary and secondary windings with theprimary winding comprising a part of the resonant circuit, a siliconcontrolled rectifier having its gate control element connected to thesecondary Winding of said saturable transformer, a load device connectedin series circuit relationship with the controlled rectifier across asource of alternating current, asecond saturable transformer havinginductively coupled primary and secondary windings with the primarywinding being connected in series circuit relationship with the loaddevice and the silicon controlled rectifier, a third saturabletransformer having inductively coupled primary and secondary Windingswith the primary winding being connected in series circuit relationshipwith the secondary winding of the second saturable transformer andblocking diode across the source of alternating current, and with thesecondary Winding of the third saturable transformer being connectedback to the gate element of the controlled rectifier through anisolating device.

References Cited by the Examiner UNITED STATES PATENTS 2,003,945 6/35Logan.

2,851,099 9/58 Snoddy 307-41 2,898,545 8/59 Bird 323-76 2,960,613 11/60Spitzer 32376 3,005,110 10/61 Elam 30741 LLOYD MCCOLLUM, PrimaryExaminer.

MILTON O. HIRSHFIELD, Examiner.

